Variable bias on Pleiades amplifiers adjusting threshold of peak limiting

For example.


On Pleiades Elektra III headphone electron tube battery powered amplifier.


The Rag resistor from anode to grid can be a 2MΩ potentiometer in series with 470KΩ. The Rag resistor makes possible the freeing of electrons to make the EF183 triode connected electron tube operate at 12V or less anode voltage.


So that bias can be adjusted for any listening volume. This can provide instatenious peak limiting making the signal more suitable to listener's ears or perception  and ordinary listening conditions.


Hearing protection is provided too by the limiting action.


The variable bias voltage may effectively adjust the threshold of limiting or in other words the power output of the amplifier for a given amount of distortion.


At small signals and near the average level where most of the music exists, the amplifier is extremely linear since it is biased in class A operation. The nearest the excursions of the signal from the operating point the more linear the electron tube is.


The user adjusts for best possible sound according to the listening conditions.


Having said the above:


While exoerimenting in practice on the Pleiades Elektra III with resistors from anode to grid ranging from 1.8MΩ to 470KΩ, there is not much difference in sound. Even with 470KΩ from anode to grid the grid potential assumes the value of -200mV from the value of -400mV when Rag is 1.8MΩ. Of course when the cathode is left to cool down the grid potential becomes that of the battery ie 12V.


Even more strange is that the sound is not much different varying the grid potential from -400mV to 400mV.


The easiest and most effective way to vary the grid potential is by changing the cathode temperature. The easiest way to do this in real time is to disconnect the heater circuit and monitor the grid potential with respect to cathode and listening to music at the same time.


More difference in power output was observed while driving a full range speaker with one EF183 through an output transformer.


More details on nearby euroelectron posts.